It is understood that owing to the lack of electrical overload and short circuit protective devices in outdoor low voltage power supply systems, damage to a transformer may result in case of short circuit during operation when the power cord is severed by a lawnmower or run over by a vehicle. To maintain the safety of such power supply systems, generally a thermal over-current circuit breaker composed of a bi-metal plate with contact points at its end is usually added in series at the secondary side of the transformer. When the bi-metal plate is heated, by the current in excess of a rated value in the circuit, the temperature of the plate and the heat stored therein reaches a specific level (the activating point) and the normally closed contact points are opened. After the heat of metal plate is dissipated, lowering the temperature to a normal temperature, the plate returns to its normal, original configuration, making the normally closed contacts close, thus restoring the electric conduction. However, the design of the above-noted prior art thermal current breaker requires additional manpower and material costs. For cost effective purposes, the applicant has developed the circuit design of the present invention, using photothermic controlling switch components.